The development of magnetically-enhanced cathodic sputtering is providing a major contribution to the coating field, because of the much higher deposition rates compared with those obtained with conventional sputtering, which make it possible to produce coatings for many industrial applications.
Magnetically-enhanced cathodic sputtering is generally carried out with either planar or cylindrical configurations, of which only the cylindrical configurations will be considered herein. The first magnetically-enhanced sputtering system to have a cylindrical configuration was a system of the Penning-type, designed for applying inside the annular interspace extending between a central cylindrical sputtering target and the substrates to be coated disposed concentrically around said central target, of an axial uniform magnetic field generated from the outside (of a Helmholtz-type coil disposed concentrically outside the vacuum bell jar). The sputtering rate increased only slightly as compared with conventional sputtering systems, taking into account the uniformity of the axial magnetic field which causes a slight intensification, but without confinement of the discharge plasma. Also, the Penning-type systems are further limited to the coating of substrates made of non-magnetic materials to avoid shielding the magnetic field generated from the outside.
The cylindrical magnetically-enhanced sputtering systems proposed herein are classified as "cylindrical magnetron sputtering" systems by U. Heisig et al. (cf. Paper "High Rate Sputtering with a Torus Plasmaton" from U. Heisig, K. Goedicke and S. Schiller--presented on 7th Int. Conf. on Electron and Ion Beam Science and Technology, Washington, D.C., USA, May 1976) or by N. Kuriyama (cf. German patent application No. 2,655,942 and U.S. Pat. No. 4,221,652). These "cylindrical magnetron sputtering" systems are designed for ensuring the application, over the sputtering face of a tubular target, of magnetic fields with flux lines forming a plurality of equiaxially spaced closed-loop toroidal arches circumferentially extending over the sputtering face, caused by a specific magnetron having a plurality of axially spaced ring magnets circumferentially exending against the back face of the target. The major advantage of such systems is the formation of a plurality of intense ring plasmas confined within these toroidal arches, which produces high sputtering rates of the tubular target. The major drawbacks of such systems involves poor target cooling and lack of uniformity of target sputtering and/or substrate coating, especially when using long targets to coat long substrates.
Proper cooling of the tubular target is rendered necessary because of excessive heating from the intense sputtering. Adequate cooling cannot be totally ensured because of the particular arrangement of the axially spaced ring magnets with respect to the tubular target, which put a severe restraint on the axial circulation of the liquid coolant along the back face of the tubular target. Also, in such a magnetron the liquid coolant generally cannot contact the annular regions of the back face of the target at the level of which the ring plasmas are confined, and so the heating is more intense. Obtainment of a uniform target sputtering and/or substrate coating appears unreliable, even with the to-and-fro axial motion of the magnetron with respect to the tubular target, because of the instability of the separate ring plasmas sustained by such a magnetron. It is known that the current-voltage characteristic of magnetrons flattens for high currents, which means that the plasma impedance decreases with increasing current. The design of a magnetron intended to sustain a plurality of separate confined plasmas constitutes a design very sensitive to the occurrence of any dissymetry (whether caused by the geometry of the different magnets, the strength of the magnetic field generated by these different magnets, or any other type of dissymmetry, such as the to-and-fro axial motion of the magnetron with respect to the tubular target). Separate plasmas act as electrical circuit elements in parallel and the occurrence of any dissymmetry results in a current disequilibrium between the separate plasmas acting as electrical elements in parallel. This disequilibrium deteriorates with time because the plasma impedance decreases with increasing current, thereby endangering the uniformity of target sputtering and/or substrate coating (the to-and-fro axial motion of the magnetron with respect to the tubular target being totally unable to restore such uniformity along the whole length of this target).
The German Patent application No. 2,707,144 (assigned to the Sloan Technology Corp.) describes (cf. FIGS. 18 and 19 of that application) a cylindrical magnetron sputtering system which is designed for generating over the sputtering face of a tubular target a simple closed-path magnetic tunnel, which axially extends along the full length of the tubular target while circumferentially extending along a small region of this tubular target (such single closed-path magnetic tunnel being generated by a magnet arrangement behind the back face of the tubular target having an extension similar to that of the magnetic tunnel to be generated). This system requires the complete rotation of the magnet arrangement by at least one turn, so as to ensure the uniform sputtering of the whole target. The complete rotation also results in lower sputtering rates. No information is given concerning the target cooling.